The present invention relates to the preparation of epoxides by the oxidation of olefins using molecular oxygen with the addition of an aldehyde additive, characterized in that the oxidation is performed in compressed carbon dioxide as the solvent.
The preparation of epoxides by the oxidation of olefins is a technically important process of great economical significance (cf. Rxc3x6mpp-Chemie Lexikon, Georg Thieme Verlag Stuttgart-New York 1997, 10th edition, Volume 2, 1186). From an ecological and economical point of view, the use of molecular oxygen as the oxidant (in the form of pure oxygen or atmospheric oxygen) is of particular interest. This is achieved, for example, by the addition of aldehyde additives to reaction mixtures consisting of the substrate and a suitable liquid organic solvent, wherein the presence of expensive transition metal catalysts which are difficult to prepare is usually required, however (Bull. Chem. Soc. Jpn. 1995, 68, 17-35, and Chem. Commun. 1997, 69-70).
The oxidation of cyclohexene to form epoxycyclohexane with molecular oxygen with the addition of an aldehyde additive in CH2Cl2 as the solvent was described with a simple catalyst which can be prepared from iron powder and catalytic amounts of a carboxylic acid (preferably acetic acid) (J. Am. Chem. Soc. 1992, 114, 7913-7914). In the application EP 0 540 009 A1, it is described that a catalyst can even be completely dispensed with under such conditions. However, the use of large amounts of the solvent CH2Cl2, which is ecologically and toxicologically dangerous, is a considerable drawback of these reactions. Our own examinations (Examples 12 to 15) have shown that CH2Cl2 in this process cannot be easily replaced by other solvents which are less dangerous, such as toluene, because significantly lower yields and side reactions through oxidation of the solvent result. In addition, the use of many conventional organic solvents requires that the explosion limits of the solvents are taken into account when molecular oxygen serves as the oxidant, which greatly limits the useful range of oxygen partial pressures.
Thus, it has been the object of the present invention to find a suitable solvent for the epoxidation of olefins with molecular oxygen with the addition of an aldehyde additive, wherein the solvent should be stable towards oxidation, reduce the danger of explosion and be toxicologically and ecologically safe.
We have found that compressed (liquid or supercritical) carbon dioxide ideally meets these requirements, and we here describe a method for the preparation of epoxides of general formula II (see below) by the oxidation of olefins of general formula I (see below) using molecular oxygen with the addition of an aldehyde additive of general formula III (see below), characterized in that the oxidation is performed in compressed carbon dioxide as the solvent.
As said olefins, substrates of general formula I can be employed 
wherein R1, R2, R3 and R4 may be the same or different, can be independently selected and may comprise a hydrogen atom or straight or branched chain, cyclic or aromatic C1-C20 hydrocarbons in which one or more carbons may be replaced by heteroatoms or/and which may be substituted with halogen, hydroxy, alkoxy, phenoxy, acyloxy, acyl, alkoxycarbonyl or phenoxycarbonyl groups, wherein the residues R1, R2, R3 or R4 may be interconnected to form rings or fused ring systems. In the method developed by us, the olefins of general formula I are oxidized to yield the corresponding epoxides of general formula II 
wherein R1, R2, R3 and R4 are defined as for formula I. This is effected with the addition of an aldehyde additive of general formula III 
wherein R5 may be a hydrogen atom, a C1-C10 alkyl group, a halogen-substituted C1-C10 alkyl group, an aryl group, a halogen-substituted aryl group, an arylalkyl group or a halogen-substituted arylalkyl group.
In the present invention, compressed, preferably supercritical, carbon dioxide (scCO2, critical data: Tc=30.9xc2x0 C., xcfx81c=73.75 bar, xcfx81c=0.468 gxc2x7cmxe2x88x923) serves as the solvent. The total pressure is within a range of from 40 to 500 bar, preferably from 60 to 200 bar.
The reaction temperature is within a range of from 0 to 200xc2x0 C., preferably between 31xc2x0 C. and 100xc2x0 C. The reaction time is not generally limited and depends on the course of the reaction, which can be monitored by means of suitable analytical methods (e.g., GC, NMR, IR). The reaction time is usually within a range of from 1 to 48 hours.
The quantity of aldehyde employed is not particularly limited, but is usually from 0.1 to 30 mol, preferably from 1 to 10 mol, per mole of olefin.
The quantity of olefin employed is not particularly limited, but is usually from 0.1 to 50 mmol per 100 g of CO2, preferably from 0.5 to 10 mmol per 100 g of CO2.
Attention must be paid to the fact that the danger of explosion upon addition of oxygen increases as the amount of olefin increases.
The molecular oxygen can be added in a pure form or in the form of mixtures with other gases (e.g., as atmospheric oxygen). The partial pressure of the oxygen employed is not particularly limited, but is usually from 1 to 20 bar, preferably from 1 to 5 bar. The use of compressed CO2 as the solvent as described herein has the advantage, as compared to many conventional organic solvents, that CO2 as a solvent is stable towards oxidation and reduces the danger of explosion. Therefore, after dissolving the olefin and aldehyde in the amount of CO2 needed as a solvent, higher partial pressures of oxygen can be safely employed, in contrast to many conventional organic solvents.
The method described by us is also compatible with the use of known oxidation catalysts. Such catalysts may serve to increase the reaction rate or enhance selectivity. The catalysts may be employed in a solid or liquid form or in a form dissolved in the reaction medium. Typical catalysts are described in detail in Bull. Chem. Soc. Jpn. 1995, 68, 17-35, and EP 0 540 009 A1, but the method is not limited to the catalysts mentioned there.
Further, due to the fact that the solvent properties of supercritical carbon dioxide vary with pressure and temperature, a separation of the main products (epoxide) and by-products (carboxylic acid of the aldehyde additive, further oxidation products of the olefin) and unreacted educts (olefin and aldehyde) from the reaction mixture is possible when the external parameters are selected appropriately. Known methods in which scCO2 is used for material separation are described in Angew. Chem. Int. Ed. EngI. 1978, 17, 702, and in M. McHugh, V. Krukonis, Supercritical Fluid Extraction, Butterworth Publishers, 1986. When a catalyst is additionally employed, its separation is also possible.